Direction from angle

BlitzMax Forums/BlitzMax Programming/Direction from angle

MattVonFat

(Posted 2005) [#1]

Is there a way to work out the increments to x and y from the angle you want to head? So if i wanted my little ant to go 90 degrees he would move 1 along x and 0 along y. I thought Sin would have something to do with it (why i dunno i just remebered using angles with it :P).

Could anyone help?

rdodson41

(Posted 2005) [#2]

Here you go:

Local angle, speed
Local xrate, yrate
Local x, y

angle = 150
speed = 5

xrate = speed * Sin( angle )
yrate = speed * Cos( angle )

x = x + xrate
y = y + yrate

Angle is of course the angle of the direction of your ant. Speed is how fast your ant is going. Xrate and Yrate are the rate that x and y are changed by. They are calculated as above and added to their respective variables.

This is how a circle is calculated. If you did a For angle = 0 to 360 sort of thing, it would calculate a circle that has a radius of 5, because a circle is calculated X = Radius * Sin( Degree ), and Y = Radius * Cos( Degree ). So in reality you are calculating a circle for one specific degree.

Hope it helps.

<EDIT> I think there is a sample of Asteroids that comes with BMX if not you could try B+. Look at those examples and you can see how they do it.

Robert

(Posted 2005) [#3]

Sin(angle) returns the "x portion" of a movement in direction <angle> of length 1. Cos(angle) returns the "y portion" of a movement in direction <angle> of length 1. If you are moving by a different distance, multiply the result by the distance.

Eg, to move 45 degrees for 5 units:
x=5*sin(45)

Small example to demonstrate:

Type Vector
Field x
Field y
End Type

Graphics 800,600,0

Local q:Vector=New Vector
q.x=100
q.y=100

Local p:Vector=q

Local direction:Float=0

While Not KeyHit(KEY_ESCAPE)

DrawLine q.x,q.y,p.x,p.y,True

q=p
p=move(p,direction,5)

If KeyDown(KEY_SPACE) direction :+ 5

Delay 20

Flip

End While

Function move:Vector(initialPos:Vector,direction:Float,distance:Float)

Local result:Vector=New Vector

result.x=initialPos.x+(distance*Sin(180-direction))
result.y=initialPos.y+(distance*Cos(180-direction))

Return result

End Function

The above code does what you need. Hold down the SPACE key whilst the line is being plotted to change the direction.

MattVonFat

(Posted 2005) [#4]

Thanks very much! This is really going to help. I guess i'll have to start reading more about these different mathematical things!

ImaginaryHuman

(Posted 2005) [#5]

I would've told you about using the bresenhams line technique which uses integers to follow a line, but with the speed of modern computers it's not only a lot easier but possibly also less code and just as fast to do it with floating point numbers.

Diordna

(Posted 2005) [#6]

Hmm, I've always used cos for x and sin for y, because it's correct according to my math teacher and the sin/cos circle thingy (the Unit Circle? I forget its name.) That way, 0 faces to the right like it should, and increasing angles go clockwise.

FlameDuck

(Posted 2005) [#7]

Hmm, I've always used cos for x and sin for y, because it's correct according to my math teacher and the sin/cos circle thingy (the Unit Circle? I forget its name.)

Me too. The problem is that on a computer the second axis (y) is inverted compared to traditional trigonometry. Thus the y coordinate for computers would be -cos(angle).

That's why most people just reverse x and y orientation, otherwise you get a lot of "interesting" results when trying to use it with ATan2 and RotateImage (which uses angles like in navigation, not trigonometry).

That way, 0 faces to the right like it should, and increasing angles go clockwise.

Except increasing angles are supposed to go counter-clockwise!